W123 transmission

[Q45Denver]

I just wanted to run this by the experts before I took any further steps. I recently purchased a 1982 300TD with about 200K miles.
There was no record for recent transmission service. The transmission was not shifting out of second and slipping badly. I adjusted the t-handle on the vacuum modulator (two turns clockwise) and now it seems to shift OK. I also drained the pan and replaced the filter (couldn't get at the torque convertor drain bolt). The fluild was dark and smelled burned. Obviously the car had been driven for some time with the transmission slipping. Also the filter had a large quantity of carbon like flakes in it but there was no evidince of any metal in it or in the pan. Is it likely that I will need a new transmission soon?
Were the carbon flakes I found from the clutch linings. What kind of service could I expect form a rebuilt transmission?
Thanks for your help.